DES算法 (转)

The Data Encryption Standard (DES) algorithm, adopted by the U.S. government in 1977, is a block cipher that transforms 64-bit data blocks under a 56-bit secret key, by means of permutation and substitution. It is officially described in FIPS PUB 46. The DES algorithm is used for many applications within the government and in the private sector.

This is a tutorial designed to be clear and compact, and to provide a newcomer to the DES with all the necessary information to implement it himself, without having to track down printed works or wade through C source code. I welcome any comments. Matthew Fischer <Mailto:mfischer@heinous.isca.uiowa.edu">mfischer@heinous.isca.uiowa.edu>

Here's how to do it, step by step:

1 Process the key.

1.1 Get a 64-bit key from the user. (Every 8th bit is considered a parity bit. For a key to have correct parity, each byte should contain an odd number of "1" bits.)

1.2 Calculate the key schedule.

1.2.1 PerfoRM the following permutation on the 64-bit key. (The parity bits are discarded, reducing the key to 56 bits. Bit 1 of the permuted block is bit 57 of the original key, bit 2 is bit 49, and so on with bit 56 being bit 4 of the original key.)

This has been a description of how to use the DES algorithm to encrypt one 64-bit block. To decrypt, use the same process, but just use the keys K[i] in reverse order. That is, instead of applying K[1] for the first iteration, apply K[16], and then K[15] for the second, on down to K[1].

To encrypt or decrypt more than 64 bits there are four official modes (defined in FIPS PUB 81). One is to go through the above-described process for each block in succession. This is called Electronic Codebook (ECB) mode. A stronger method is to exclusive-or each plaintext block with the preceding ciphertext block prior to encryption. (The first block is exclusive-or'ed with a secret 64-bit initialization vector (IV).) This is called Cipher Block Chaining (CBC) mode. The other two modes are Output Feedback (OFB) and Cipher Feedback (CFB).

When it comes to padding the data block, there are several options. One is to simply append zeros. Two suggested by FIPS PUB 81 are, if the data is binary data, fill up the block with bits that are the opposite of the last bit of data, or, if the data is ASCII data, fill up the block with random bytes and put the ASCII character for the number of pad bytes in the last byte of the block. Another technique is to pad the block with random bytes and in the last 3 bits store the original number of data bytes.

The DES algorithm can also be used to calculate checksums up to 64 bits long (see FIPS PUB 113). If the number of data bits to be checksummed is not a multiple of 64, the last data block should be padded with zeros. If the data is ASCII data, the first bit of each byte should be set to 0. The data is then encrypted in CBC mode with IV = 0. The leftmost n bits (where 16 <= n <= 64, and n is a multiple of 8) of the final ciphertext block are an n-bit checksum